Tumours of the Parietal and Occipital Lobes

Occipital tumours reveal themselves by their interference with the visual fields and the loss of field is naturally in the form of a homonymous hemianopia, at first partial, but as a rule becoming complete very rapidly; more rapidly than in the case of hemianopia from the downwards pressure of a parietal or the upward pressure of a temporal tumour upon the optic radiation. In these latter cases the hemianopia has a tendency to be at first quadrantic with the lower quadrants first affected in parietal cases and the upper quadrants in temporal cases. Of great interest in the homonymous hemianopia due to occipital tumours is the sparing of the macular area across the midline, this occurring even when the occipital lobe is completely destroyed by the tumour or completely removed by operation. There has been much speculation as to the reason for this and it is suggested that a decussation of some of the macular fibres occurs further forward and the macular region is thus bilaterally represented. The preservation of the whole macular area is often of great benefit to the patient, making his homonymous hemianopia much less of an infliction.